Fuel cell module

ABSTRACT

A fuel cell module includes a first casing containing the fuel cell stack, and a second casing containing the first casing, an exhaust gas combustor, and a heat exchanger. The exhaust gas combustor has a combustion gas discharge opening opened to a combustion gas chamber formed between the first casing and the second casing.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2014-175612 filed on Aug. 29, 2014, thecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fuel cell module including a fuelcell stack formed by stacking a plurality of fuel cells for generatingelectrical energy by electrochemical reactions of a fuel gas and anoxygen-containing gas.

2. Description of the Related Art

In general, a solid oxide fuel cell (SOFC) employs a solid electrolyte.The solid electrolyte is an oxide ion conductor such as stabilizedzirconia. The solid electrolyte is interposed between an anode and acathode to form an electrolyte electrode assembly (also referred to asMEA). The electrolyte electrode assembly is sandwiched betweenseparators (bipolar plates). In use, generally, predetermined numbers ofthe electrolyte electrode assemblies and the separators are stackedtogether to form a fuel cell stack.

As the fuel gas supplied to the fuel cell, normally, a hydrogen gasproduced from hydrocarbon raw material by a reformer has been used. Ingeneral, in the reformer, a reforming raw gas is obtained from ahydrocarbon raw fuel of a fossil fuel or the like, such as methane orLNG, and thereafter, the reforming raw gas undergoes partial oxidationreforming, steam reforming, or autothermal reforming, etc. to produce areformed gas (fuel gas).

It is desired that this fuel cell is operated relatively at hightemperature, and that the fuel cell has the heat insulating propertiesto improve the heat efficiency. A fuel cell system aimed to address thispoint has been proposed in Japanese Laid-Open Patent Publication No.2012-182032. The fuel cell system has simple and compact structure withheat insulating properties while allowing a fuel cell stack to be heateduniformly.

This fuel cell system includes a fuel cell stack including a pluralityof stacked fuel cells, a reformer, a heat exchanger, and a fuel cellstack mounting member for mounting the fuel cell stack. Further, thefuel cell system includes a fluid unit equipped with a frame member forholding the reformer, the heat exchanger, and the fuel cell stackmounting member. The fuel cell stack is provided on one side of thefluid unit, and a combustor is provided adjacent to the other side ofthe fluid unit. The combustor heats the reformer and the heat exchangerby the combustion gas produced in combustion.

Further, a first case member containing the fuel cell stack is connectedto the fuel cell stack mounting member, and a second case membercontaining the first case member and the fluid unit are connected to theframe member.

Further, both ends of a channel member are connected to the fuel cellstack mounting member and the heat exchanger. An off gas which has beenpartially consumed in the power generation reaction of the fuel cellstack is supplied from an internal space of the first case member to theheat exchanger as a heating medium through the channel member. Afterheat exchange at the heat exchanger, the off gas discharged from theheat exchanger is discharged through an off gas channel formed betweenthe first case member and the second case member.

As described above, the off gas discharged after partial consumption inthe power generation reaction flows into the heat exchanger as a heatingmedium for heat exchange with an oxygen-containing gas before theoxygen-containing gas is supplied to the fuel cell stack. According tothe disclosure, with the structure, the waste heat of the off gas can becollected effectively, and improvement in the heat efficiency isachieved easily. In particular, it becomes possible to utilize the wasteheat for directly heating the fuel cells, and effectively utilize thewaste heat as a heating energy for heating the oxygen-containing gas bythe heat exchanger.

SUMMARY OF THE INVENTION

The present invention has been made in relation to the technique of thistype, and an object of the present invention is to provide a fuel cellmodule having compact structure in which it is possible to suppressradiation of heat from the fuel cell stack as much as possible, andimprove the start-up performance and the heat efficiency.

A fuel cell module according to the present invention includes a fuelcell stack, a first casing, an exhaust gas combustor, a heat exchanger,a reformer, and a hydrodesulfurizer. The fuel cell stack includes aplurality of stacked fuel cells configured to generate electrical energyby electrochemical reactions of a fuel gas and an oxygen-containing gas.

The first casing contains the fuel cell stack. The exhaust gas combustoris configured to combust the fuel gas discharged from the fuel cellstack as a fuel exhaust gas and the oxygen-containing gas dischargedfrom the fuel cell stack as an oxygen-containing exhaust gas to producea combustion gas. The heat exchanger is configured to increase thetemperature of the oxygen-containing gas by heat exchange with thecombustion gas and supplying the oxygen-containing gas to the fuel cellstack. The reformer is configured to reform a raw fuel chieflycontaining hydrocarbon to produce the fuel gas supplied to the fuel cellstack. The hydrodesulfurizer is configured to remove a sulfur componentcontained in the raw fuel.

The fuel cell module includes a second casing containing at least thefirst casing, the exhaust gas combustor, and the heat exchanger. Theexhaust gas combustor has a combustion gas discharge opening opened to acombustion gas chamber formed between the first casing and the secondcasing.

In the present invention, the second casing contains at least the firstcasing (containing the fuel cell stack), the exhaust gas combustor, andthe heat exchanger. Further, the exhaust gas combustor has thecombustion gas discharge opening opened to the combustion gas chamberformed between the first casing and the second casing. In the structure,the hot combustion gas flows around the first casing, i.e., flows aroundthe fuel cell stack, and can function as a heat insulating layer of thefuel cell stack. Therefore, it becomes possible to suppress heatradiation from the fuel cell stack as much as possible. Thus, thedesired start-up performance and the high heat efficiency are achieved.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which preferredembodiments of the present invention are shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically showing structure of a fuel cellmodule according to a first embodiment of the present invention;

FIG. 2 is a partial cross sectional front view showing the fuel cellmodule;

FIG. 3 is a perspective view showing the fuel cell module;

FIG. 4 is an exploded perspective view showing main components of thefuel cell module;

FIG. 5 is a perspective view showing an exhaust gas combustor and a heatexchanger of the fuel cell module;

FIG. 6 is a perspective view showing a reformer and a hydrodesulfurizerof the fuel cell module; and

FIG. 7 is a diagram schematically showing structure of a fuel cellmodule according to a second embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A fuel cell module 10 according to a first embodiment of the presentinvention shown in FIG. 1 is used in various applications, includingstationary, in-vehicle, and mobile applications.

The fuel cell module 10 includes a fuel cell stack 12, a reformer 14, ahydrodesulfurizer 16, an exhaust gas combustor 18, and a heat exchanger20. An oxygen-containing gas supply apparatus 22 for supplying anoxygen-containing gas and a raw fuel supply apparatus 24 for supplying araw fuel (e.g., city gas) are connected to the fuel cell module 10.

The oxygen-containing gas supply apparatus 22 is connected to thereformer 14 through an oxygen-containing gas supply channel 26, and theraw fuel supply apparatus 24 is connected to the reformer 14 through afuel gas supply channel 28. In the illustrated embodiment, theoxygen-containing gas supply channel 26 is connected to a positionsomewhere in the fuel gas supply channel 28. Alternatively, theoxygen-containing gas supply channel 26 may be connected directly to thereformer 14. The reformer 14 performs partial oxidation reforming orsteam reforming of a raw fuel chiefly containing hydrocarbon to producea fuel gas supplied to the fuel cell stack 12.

The hydrodesulfurizer 16 is provided for the fuel gas supply channel 28at a position downstream of the reformer 14. The hydrodesulfurizer 16removes sulfur components contained in the raw fuel. The fuel cell stack12 is provided downstream of the hydrodesulfurizer 16 through a fuel gassupply channel 28 a.

A fuel exhaust gas channel 30 for discharging the fuel gas partiallyconsumed in power generation reaction as a fuel exhaust gas and anoxygen-containing exhaust gas channel 32 for discharging theoxygen-containing gas partially consumed in power generation reaction asan oxygen-containing exhaust gas are connected to the fuel cell stack12. The fuel exhaust gas channel 30 and the oxygen-containing exhaustgas channel 32 are connected to an inlet side of the exhaust gascombustor 18, and an inlet side of the heat exchanger 20 is connected toan outlet side of the exhaust gas combustor 18.

The oxygen-containing gas (air) is supplied from the oxygen-containinggas supply apparatus 22 to the heat exchanger 20 through anoxygen-containing gas supply channel 26 a. The oxygen-containing gassupply channels 26, 26 a are merged through a control valve 34 capableof controlling the flow rate of the oxygen-containing gas. After theheat exchanger 20 heats the oxygen-containing gas by heat exchange witha combustion gas, the heat exchanger 20 supplies the heatedoxygen-containing gas to the fuel cell stack 12 through anoxygen-containing gas supply channel 36.

A recycling channel 38 is connected to a position somewhere in the fuelexhaust gas channel 30. The recycling channel 38 circulates some of thefuel exhaust gas to return to a position of the fuel gas supply channel28, the position provided upstream of the reformer 14 and thehydrodesulfurizer 16. A control valve 40 is provided at a divergingsection of the recycling channel 38 and the fuel exhaust gas channel 30,for controlling the flow rate of the fuel exhaust gas flowing throughthe recycling channel 38. An ejector 42 is provided at a merging sectionof the recycling channel 38 and the fuel gas supply channel 28, forcontrolling the flow rate of the fuel exhaust gas flowing through therecycling channel 38 by the negative pressure resulting from the supplyof the raw fuel to the reformer 14.

The fuel cell stack 12 includes a plurality of solid oxide fuel cells 44for generating electrical energy by electrochemical reactions of thefuel gas (mixed gas of hydrogen, methane, and carbon monoxide) and theoxygen-containing gas (air). The fuel cells 44 are stacked together in avertical direction or a horizontal direction. For example, each of thefuel cells 44 includes an electrolyte electrode assembly (MEA) 51. Theelectrolyte electrode assembly 51 includes a cathode 48, an anode 50,and an electrolyte 46 interposed between the cathode 48 and the anode50. The electrolyte 46 is an ion oxide conductor made of, e.g.,stabilized zirconia.

A cathode side separator 52 and an anode side separator 54 are providedon both sides of the electrolyte electrode assembly 51. The cathode sideseparator 52 has an oxygen-containing gas flow field 56 for supplyingthe oxygen-containing gas to the cathode 48, and the anode sideseparator 54 has a fuel gas flow field 58 for supplying the fuel gas tothe anode 50. As the fuel cell 44, various types of conventional SOFCscan be adopted.

The operating temperature of the fuel cell 44 is high, at severalhundred ° C. Methane in the fuel gas is reformed at the anode 50 toproduce hydrogen and CO, and the hydrogen and CO are supplied to aportion of the electrolyte 46 adjacent to the anode 50.

The fuel cell stack 12 has an oxygen-containing gas supply passage 60 aconnected to an inlet side of each oxygen-containing gas flow field 56,and an oxygen-containing gas discharge passage 60 b is connected to anoutlet side of each oxygen-containing gas flow field 56. Theoxygen-containing gas supply channel 36 is connected to theoxygen-containing gas supply passage 60 a, and the oxygen-containingexhaust gas channel 32 is connected to the oxygen-containing gasdischarge passage 60 b.

Further, the fuel cell stack 12 has a fuel gas supply passage 62 aconnected an inlet side of each fuel gas flow field 58, and a fuel gasdischarge passage 62 b connected to an outlet side of each fuel gas flowfield 58. The fuel gas supply channel 28 a is connected to the fuel gassupply passage 62 a, and the fuel exhaust gas channel 30 is connected tothe fuel gas discharge passage 62 b.

Basically, the reformer 14 functions as a partial oxidation reformer(PDX) at the time of start-up operation, and functions as an autothermalreformer (AR) at the time of rated operation. In partial oxidationreforming, by partial oxidation reforming reaction of theoxygen-containing gas and the raw fuel chiefly containing hydrocarbon(e.g., city gas), the raw fuel is reformed to produce the fuel gas.

Specifically, the partial oxidation reformer is a preliminary reformerfor reforming higher hydrocarbon (C₂₊) such as ethane (C₂H₆), propane(C₃H₈), and butane (C₄H₁₀) in the raw fuel to produce the fuel gaschiefly containing hydrogen and CO by partial oxidation reforming.Partial oxidation reforming is performed at the operating temperature ina range of about 500° C. to 1000° C. The partial oxidation reforminguses at least one catalytic metal selected from Pt (platinum) Rh(rhodium), and Pd (palladium).

In autothermal reforming, partial oxidation reforming and steamreforming are performed in combination. In this steam reforming, themixed gas of the raw fuel and water vapor is reformed to produce thefuel gas. Steam reforming uses at least one catalytic metal selectedfrom Ru (ruthenium), Ni, (nickel), Pt (platinum), Rh (rhodium), Pd(palladium), Ir (iridium), and Fe (iron). In the reformer 14, at leastone catalytic metal selected from Pt (platinum), Rh (rhodium), and Pd(palladium) is used as a single catalyst metal for performing partialoxidation reforming and steam reforming.

The hydrodesulfurizer 16 induces reaction of sulfur compound withhydrogen on catalyst (Ni—Mo based catalyst, Co—Mo based catalyst) totransform them into hydrogen sulfide, and takes the hydrogen sulfideinto zinc oxide for removal. For example, the following reaction occurs.

H₃C—S—CH₃+2H₂→2CH₄+H₂S

H₂S+ZnO→H₂O+ZnS

The exhaust gas combustor 18 combusts the fuel gas discharged from thefuel cell stack 12 as the fuel exhaust gas and the oxygen-containing gasdischarged from the fuel cell stack as the oxygen-containing exhaust gasto produce the combustion gas, and supplies the combustion gas to theheat exchanger 20. The heat exchanger 20 heats the oxygen-containing gasby heat exchange with the combustion gas, and supplies theoxygen-containing gas to the fuel cell stack 12.

As shown in FIGS. 2 and 3, the fuel cell module 10 includes a firstcasing 64 containing the fuel cell stack 12 and a second casing 66containing at least the fuel cell stack 12, the exhaust gas combustor 18and the heat exchanger 20. A combustion gas chamber 68 is formed betweenthe first casing 64 and the second casing 66.

In the second casing 66, the exhaust gas combustor 18 and the heatexchanger 20 are provided integrally below the fuel cell stack 12, and acombustion gas discharge port 70 is formed at an upper position of thesecond casing 66 for discharging the combustion gas in the combustiongas chamber 68 to the outside.

As shown in FIGS. 4 and 5, the exhaust gas combustor 18 is coupled to anend of the heat exchanger 20. The exhaust gas combustor 18 has arectangular parallelepiped shape, and has an internal combustion chamber72. The combustion chamber 72 is connected to the fuel exhaust gaschannel 30 and the oxygen-containing exhaust gas channel 32. Theoxygen-containing exhaust gas and the fuel exhaust gas are supplied tothe combustion chamber 72 for combustion in the combustion chamber 72.

For example, combustion catalyst comprising metal or aluminum supportssupporting Pt (platinum), Rh (rhodium), or Pd (palladium) is placed inthe combustion chamber 72. An ignition device 74 a is provided in thecombustion chamber 72, for applying heat from the outside when theexhaust gas temperature does not reach the combustion temperature.

The heat exchanger 20 has a rectangular shape, and includes a pluralityof combustion gas flow channels 76. The exhaust gas combustor 18 isconnected to one end of the heat exchanger 20. The combustion gas flowchannels 76 are connected to the combustion chamber 72 of the exhaustgas combustor 18. The combustion gas flows along each of the combustiongas flow channels 76 in the direction indicated by the arrow A. Thecombustion gas flow channels 76 are connected to a combustion gasdischarge chamber 78.

A plurality of combustion gas discharge openings 80 are formed at upperpositions of the combustion gas discharge chamber 78, and the combustiongas discharge openings 80 are opened to the internal space of thecombustion gas chamber 68. As shown in FIGS. 2 and 3, in the combustiongas chamber 68, the combustion gas discharge openings 80 and thecombustion gas discharge port 70 are provided at diagonal positions. Anignition device 74 b is provided in the combustion gas discharge chamber78, for achieving desired temperature of the combustion gas which isdischarged to the combustion gas chamber 68 (see FIGS. 4 and 5).

As shown in FIG. 5, in the heat exchanger 20, a plurality ofoxygen-containing gas flow channels 82 are provided for allowing theoxygen-containing gas to flow in a direction indicated by an arrow B(opposite to the direction indicated by the arrow A). The combustion gasflow channels 76 and the oxygen-containing gas flow channels 82 areprovided alternately. The ends of the oxygen-containing gas flowchannels 82 on the inlet side are connected to the oxygen-containing gassupply channel 26 a, and the ends of the oxygen-containing gas flowchannels 82 on the outlet side are connected to the oxygen-containinggas supply channel 36.

As shown in FIGS. 2 and 3, both of the reformer 14 and thehydrodesulfurizer 16 are provided integrally outside the combustion gaschamber 68 at the bottom of the second casing 66. As shown in FIGS. 4and 6, the reformer 14 is provided inside the hydrodesulfurizer 16. Thereformer 14 has a rectangular parallelepiped shape. The fuel gas supplychannel 28 is connected to a bottom of the reformer 14 near one endthereof. Catalytic metal (not shown) fills the reformer 14, and anopening 84 is formed on a side portion at the other end of the reformer14. An ignition device 74 c is provided in the reformer 14, forincreasing the temperature of the reformer 14.

The opening 84 of the reformer 14 is connected to one internal end ofthe hydrodesulfurizer 16. The reformer 14 is provided at a substantiallycentral position in the hydrodesulfurizer 16, and a hydrodesulfurizationchamber 86 is provided around the reformer 14. Though not shown,catalyst fills the hydrodesulfurization chamber 86. The fuel gas supplychannel 28 a is connected to the other internal end of thehydrodesulfurizer 16 through an opening 88.

Operation of this fuel cell module 10 will be described below.

At the time of start-up operation of the fuel cell module 10, as shownin FIG. 1, air as the oxygen-containing gas is supplied from theoxygen-containing gas supply apparatus 22 to the oxygen-containing gassupply channels 26, 26 a. Some of the air is merged into the fuel gassupply channel 28, and supplied to the reformer 14. The remaining air issupplied to the heat exchanger 20.

In the meanwhile, a raw fuel such as the city gas (containing CH₄, C₂H₆,C₃H₈, C₄H₁₀) is supplied from the raw fuel supply apparatus 24 to thefuel gas supply channel 28. The raw fuel is supplied into the reformer14. Therefore, the mixed gas of the raw fuel and the air is suppliedinto the reformer 14, and the mixed gas is ignited by the ignitiondevice 74 c to start partial oxidation reforming.

For example, if O₂/C=0.5, partial oxidation reaction (2CH₄+O₂→4H₂+2CO)is induced. This partial oxidation reaction is exothermic reaction. Areducing gas (fuel gas) at high temperature (about 500° C. to 1000° C.)is produced from the reformer 14. The hot reducing gas is supplied tothe hydrodesulfurizer 16. Specifically, as shown in FIG. 6, after thereducing gas flows in the longitudinal direction of the reformer 14, thereducing gas flows through the opening 84 into the hydrodesulfurizationchamber 86. Therefore, sulfur compound contained in the reducing gas istransformed into hydrogen sulfide, and then, the hydrogen sulfide istaken into zinc oxide for removal.

After the sulfur compound is removed from the reducing gas, the reducinggas flows through the fuel gas supply channel 28 a, and the reducing gasis supplied to the fuel gas supply passage 62 a of the fuel cell stack12. As shown in FIG. 1, in the fuel cell stack 12, after the hotreducing gas flows through each fuel gas flow field 58, the reducing gasis discharged from the fuel gas discharge passage 62 b to the fuelexhaust gas channel 30. Some of the reducing gas is diverged into therecycling channel 38, and the remaining reducing gas flows into theexhaust gas combustor 18 connected to the fuel exhaust gas channel 30.

As shown in FIG. 5, as described later, air (oxygen-containing exhaustgas) is supplied to the combustion chamber 72 of the exhaust gascombustor 18, the air and the reducing gas are self-ignited, or ignitedby the ignition device 74 a, and combusted. The combustion gas producedin the combustion chamber 72 flows through the plurality of combustiongas flow channels 76 of the heat exchanger 20, and then, the combustiongas is discharged into the combustion gas discharge chamber 78. Further,the combustion gas flows from the combustion gas discharge openings 80into the combustion gas chamber 68.

In the heat exchanger 20, the air is supplied from the oxygen-containinggas supply channel 26 a to the oxygen-containing gas flow channels 82.When the air flows along each of the oxygen-containing gas flow channels82, the air is heated by heat exchange with the combustion gas. As shownin FIG. 1, the heated air is supplied to the oxygen-containing gassupply passage 60 a of the fuel cell stack 12 through theoxygen-containing gas supply channel 36, and the fuel gas is dischargedto the outside of the fuel cell module 10 as an exhaust gas.

After this air flows through each oxygen-containing gas flow field 56,the air is discharged from the oxygen-containing gas discharge passage60 b to the oxygen-containing exhaust gas channel 32. Further, the airflows into the exhaust gas combustor 18, and the air is used for thecombustion process. Therefore, by combustion in the exhaust gascombustor 18, the fuel cell stack 12 is heated by the radiated heat ortransmitted heat.

In the meanwhile, the fuel exhaust gas diverged into the recyclingchannel 38 flows into the fuel gas supply channel 28 under the negativepressure operation of the ejector 42, and the fuel exhaust gas issupplied to the reformer 14. The fuel exhaust gas contains hydrogen andwater vapor, and the water vapor is sent to the reformer 14 to performsteam reforming. Thus, since the air and the water vapor are supplied tothe reformer 14, partial oxidation reforming and steam reforming areperformed in the reformer 14, and also the reformer 14 performsautothermal reforming function.

At the time of power generation operation of the fuel cell stack 12, asin the case of the start-up operation of the fuel cell stack 12, the airflows through the oxygen-containing gas flow field 56, and the fuel gasflows through the fuel gas flow field 58. Thus, the air is supplied tothe cathode 48 of each fuel cell 44, and the fuel gas is supplied to theanode 50 of each fuel cell 44 to generate electrical energy by chemicalreactions.

In the first embodiment, as shown in FIGS. 2 and 3, the fuel cell stack12 is placed in the first casing 64, and the second casing 66 containsthe first casing 64, the exhaust gas combustor 18, and the heatexchanger 20. Further, the exhaust gas combustor 18 has the combustiongas discharge openings 80 opened to the combustion gas chamber 68 formedbetween the first casing 64 and the second casing 66.

In the structure, the hot combustion gas discharged from the combustiongas discharge openings 80 of the exhaust gas combustor 18 to thecombustion gas chamber 68 flows around the first casing 64, i.e., flowsaround the fuel cell stack 12, and functions as a heat insulating layerof the fuel cell stack 12. Therefore, it becomes possible to suppressheat radiation from the fuel cell stack 12 as much as possible. Thus,the desired start-up performance and the high efficiency are achieved.

Further, the exhaust gas combustor 18 and the heat exchanger 20 areprovided below the first casing 64, and the combustion gas dischargeport 70 is formed at the upper portion of the second casing 66, fordischarging the combustion gas flowing through the combustion gaschamber 68. In this regard, the combustion gas discharge port 70 and thecombustion gas discharge openings 80 are provided at diagonal positionsacross the first casing 64.

In the structure, the combustion gas flows from the lower position ofthe first casing 64, and flows around the first casing 64. Then, thecombustion gas is discharged from the upper position of the secondcasing 66. Thus, the hot combustion gas flows around the fuel cell stack12, and functions as the heat insulating layer of the fuel cell stack12. Therefore, it becomes possible to suppress heat radiation from thefuel cell stack 12 as much as possible. Accordingly, improvement in thestart-up performance and the high efficiency is achieved.

Further, the reformer 14 and the hydrodesulfurizer 16 are providedoutside, and below the second casing 66. In the structure, the reformer14 and the hydrodesulfurizer 16 are not exposed to the hot atmosphere inthe second casing 66. Accordingly, it is possible to suitably achievethe desired reforming efficiency and desulfurization efficiency.

Further, as shown in FIG. 1, the fuel cell module 10 includes the fuelgas supply channel 28 having the reformer 14 and the hydrodesulfurizer16, for supplying the fuel gas to the fuel cell stack 12. In thisregard, the fuel exhaust gas is discharged from the fuel cell stack 12into the fuel exhaust gas channel 30, and the recycling channel 38 isconnected to the fuel exhaust gas channel 30, for circulating some ofthe fuel exhaust gas to return to an upstream position of the reformer14 in the fuel gas supply channel 28.

Accordingly, improvement in the fuel utilization ratio and the powergeneration efficiency is achieved. Further, the fuel exhaust gas chieflycontains the unconsumed fuel gas and the water vapor. Thus, bycirculating the unconsumed fuel gas, in the hydrodesulfurizer 16, itbecomes possible to perform hydrodesulfurization. Moreover, bycirculating the water vapor, in the reformer 14, it becomes possible toperform autothermal reforming utilizing partial oxidation reforming andsteam reforming in combination. Thus, the desired start-up performanceof partial oxidation reforming and the high efficiency of steamreforming are achieved.

Further, the ejector 42 is provided at the merging section of therecycling channel 38 and the fuel gas supply channel 28 for controllingthe flow rate of the fuel exhaust gas flowing through the recyclingchannel 38 by the negative pressure resulting from the supply of rawfuel to the reformer 14. Therefore, it becomes possible to adjustrecycling of the fuel exhaust gas in correspondence with the amount ofelectrical energy generated in power generation of the fuel cells 44.Accordingly, improvement of the fuel utilization ratio and improvementof the power generation efficiency are achieved.

Further, the control valve 40 is provided at the diverging section ofthe recycling channel 38 and the fuel exhaust gas channel 30, forcontrolling the flow rate of the fuel exhaust gas flowing through therecycling channel 38.

Therefore, it becomes possible to finely adjust recycling of the fuelexhaust gas in correspondence with the amount of electrical energygenerated in power generation of the fuel cells 44. Thus, improvement ofthe fuel utilization ratio and improvement of the power generationefficiency are achieved.

Further, the fuel cell module 10 is a solid oxide fuel cell module.Therefore, the fuel cell module 10 is optimally applicable to hightemperature type fuel cells such as SOFC.

FIG. 7 is a diagram schematically showing structure of a fuel cellmodule 90 according to a second embodiment of the present invention. Theconstituent elements that are identical to those of the fuel cell module10 according to the first embodiment are labeled with the same referencenumerals and detailed description thereof will be omitted.

The fuel cell module 90 has a reformer 14 a and a hydrodesulfurizer 16a, and the reformer 14 a is provided for the fuel gas supply channel 28,downstream of the hydrodesulfurizer 16 a.

In the second embodiment, the same advantages as in the case of thefirst embodiment are obtained.

While the invention has been particularly shown and described with areference to preferred embodiments, it will be understood thatvariations and modifications can be effected thereto by those skilled inthe art without departing from the scope of the invention as defined bythe appended claims.

What is claimed is:
 1. A fuel cell module comprising: a fuel cell stackincluding a plurality of stacked fuel cells configured to generateelectrical energy by electrochemical reactions of a fuel gas and anoxygen-containing gas; a first casing containing the fuel cell stack; anexhaust gas combustor configured to combust the fuel gas discharged fromthe fuel cell stack as a fuel exhaust gas and the oxygen-containing gasdischarged from the fuel cell stack as an oxygen-containing exhaust gasto produce a combustion gas; a heat exchanger configured to increase atemperature of the oxygen-containing gas by heat exchange with thecombustion gas and supplying the oxygen-containing gas to the fuel cellstack; a reformer configured to reform a raw fuel chiefly containinghydrocarbon to produce the fuel gas supplied to the fuel cell stack; anda hydrodesulfurizer configured to remove a sulfur component contained inthe raw fuel, wherein the fuel cell module includes a second casingcontaining at least the first casing, the exhaust gas combustor, and theheat exchanger; and the exhaust gas combustor has a combustion gasdischarge opening opened to a combustion gas chamber formed between thefirst casing and the second casing.
 2. The fuel cell module according toclaim 1, wherein the exhaust gas combustor and the heat exchanger areprovided below the first casing; a combustion gas discharge port isformed at an upper position of the second casing and is configured todischarge the combustion gas flowing through the combustion gas chamber;and the exhaust gas discharge port and the combustion gas dischargeopening are provided at diagonal positions across the first casing. 3.The fuel cell module according to claim 1, wherein the reformer and thehydrodesulfurizer are provided outside and below the second casing. 4.The fuel cell module according to claim 1, further comprising a fuel gassupply channel having the reformer and the hydrodesulfurizer and beingconfigured to supply the fuel gas to the fuel cell stack; wherein thefuel exhaust gas is discharged from the fuel cell stack into a fuelexhaust gas channel, and a recycling channel is connected to the fuelexhaust gas channel and is configured to circulate some of the fuelexhaust gas to return to a position of the fuel gas supply channel, theposition provided upstream of the reformer and the hydrodesulfurizer. 5.The fuel cell module according to claim 4, wherein an ejector isprovided at a merging section of the recycling channel and the fuel gassupply channel and is configured to control a flow rate of the fuelexhaust gas flowing through the recycling channel by negative pressureresulting from supply of the raw fuel to the reformer or thehydrodesulfurizer.
 6. The fuel cell module according to claim 4, whereina control valve is provided at a diverging section of the recyclingchannel and the fuel exhaust gas channel and is configured to control aflow rate of the fuel exhaust gas flowing through the recycling channel.7. The fuel cell module according to claim 1, wherein the fuel cellmodule is a solid oxide fuel cell module.